In timing certain operations it is necessary to provide reasonably accurate timing for short and long intervals. An example of such a requirement would be a microwave oven where the cycle time may be as short as ten seconds or as long as 35 minutes. With the usual timer arrangement having a linear-type dial, i.e. equal angular movement in equal time periods, setting an interval as short as ten seconds in a dial having 35-minute capacity becomes a near impossibility. Various solutions have been proposed to overcome this type of an arrangement. One such solution is a digital counter employing counter wheels. This type of counter is expensive. Another solution would be to provide two timers, one for the full 35 minutes and the other for a 5-minute program. Another solution employs the use of non-circular gearing causing the long time settings to be compressed while expanding the short time settings. A difficulty in such an arrangement is the high manual torque required to set the timer due to the high adverse gear ratios encountered with the non-circular gearing coupled with the drag of the gear reduction from the motor. A friction clutch in the reduction gearing does not overcome the objection.
Timers such as are used in this type of appliance customarily are provided with a bell to signal the end of the timing cycle. Bell ringing requires an impact on the bell and immediate withdrawal of the striker so the bell will ring rather than be damped and give a dead sound. The usual solution in the past has been to mount the striker on the end of a cantilevered spring which at the rest position would be spaced slightly from the bell. If this spacing gets too great, insufficient impact is obtained and the bell is weak sounding. As the spacing is decreased the criticality of adjustment increases and this directly results in higher cost manufacture.
In order to obtain a good strike action on the bell it is necessary that the cam follower which actuates the striker move with a snap action but this in prior designs has resulted in too high a torque to manually drive the timer out of the zero position. The very geometry which gives the snap action worked directly against easy movement out of the zero position.